Catedral de León

Milan Cathedral was the biggest, Chartres had the most inspiring stained-glass pictures, Palma de Majorca had the largest rose window, and so on. But León—at the time the leading city of Christian Spain—set the record for the highest proportion of window space. Its stained-glass windows occupy 1,672 sq. m (18,000 sq. ft.), or almost all the space where you’d expect the walls to be; they soar 34m (112 ft.) to the vaulted ceiling, framed by the slenderest of columns. It’s a wondrous feat of engineering—but all your kids will notice is that the interior glows with color, like no place else on earth.

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The cathedral’s 13th-century architects, Juan Pérez and Maestro Enrique, were, in effect, precursors of Mies van der Rohe, with one hitch—they didn’t have steel girders to support their glass walls. And while it has stood for 7 centuries, the cathedral contains some 125 original stained-glass windows, plus 57 oculi, and their weight strains the walls. The job of supporting the roof (its delicate vaulting was added in the Renaissance) was taken over by flying buttresses on the exterior; the walls have all they can do just to hold the glass.

An air of dignified calm pervades the cathedral’s cloisters, full of faded frescoes and Romanesque and Gothic tombs; a short walk northwest of the cathedral you can visit a splendidly decorated Romanesque church, Pantéon y Museos de San Isidoro, Plaza San Isidoro 4, which contains the tombs of 23 Leónese kings. Surrounding the two churches, León’s historic quarter is also worth exploring, as you’re already here. But the reason to come is those glorious windows; everything else in town literally pales in comparison.